Nutrient availability is one of the main factors affecting plant growth and development. Nutrient management, including the application of fertilizers, is crucial for optimal productivity in commercial crop production. Many nutrients, including both mineral and non-mineral elements, are essential for a plant's growth and survival. The non-mineral elements can include, for example, hydrogen, oxygen, and carbon, typically available from the surrounding air and water. The mineral nutrients, including nitrogen (N), phosphorous (P), and potassium (K) are available or made available in the soil for uptake by the plant's roots.
The mineral nutrients can generally be divided into two groups: macronutrients, including primary nutrients and secondary nutrients, and micronutrients. The primary mineral nutrients include N, P, and K. Large amounts of these nutrients are essential to a plant's survival, and therefore typically make up the majority of a fertilizer composition. In addition to primary nutrients, secondary nutrients are required in much smaller amounts than those of the primary nutrients. Secondary nutrients include, for example, calcium (Ca), sulfur (S), and magnesium (Mg).
Micronutrients can include, for example, boron (B), copper (Cu), iron (Fe), manganese (Mn), molybdenum (Mo), zinc (Zn), chlorine (Cl), cobalt (Co), sodium (Na), and combinations thereof. Despite being present in trace quantities, generally in concentrations less than 100 parts per million (ppm) in plant tissues, micronutrients are essential for carrying out a wide range of physiological functions, including photosynthesis, chlorosis, metabolic regulation, and osmotic regulation. However, micronutrient deficiencies are common in soils throughout the world, partly due to the fact that many micronutrients are easily adsorbed or precipitated in soil, which compromises their solubility and availability. The steady growth of crop yields during recent decades has compounded the problem by progressively depleting soil micronutrient pools.
Zinc deficiency appears to be a frequent micronutrient deficiency problem in crops worldwide, particularly in countries where soils are low in plant available Zn. It is especially common in soils with high pH (i.e., alkaline soils) and low in organic matter. Soil acidity influences the availability of Zn more than any other factor, with lower Zn solubility as soil pH increases. The availability of Zn may also be reduced by water logging and where root growth is restricted. Cool wet weather, low light intensity, and/or high soil nitrogen, phosphorus, or copper may intensify Zn deficiency.
To address Zn deficiencies, efforts in the agricultural community have been directed at applying Zn exogenously to soil or to fertilizers. Applying Zn directly to the soil is inefficient because Zn, like other micronutrients, is generally present as positively charged metal ions and will readily be strongly sorbed to soil minerals and organic matter, and/or react with negatively charged phosphate (H2PO42− and HPO4−), carbonate (CO32−) and/or hydroxide ions (OH), ultimately forming new compounds that are not available to plants. All these ions are abundant in soil and soilless growth media.
Efforts at applying Zn and other micronutrients to fertilizers have been more promising. For example, in Australian Patent No. AU 554,749, Zn deficiency was addressed by developing a process for adhering Zn to a phosphate-containing fertilizer by treatment with mineral acid in order to bind the Zn compound to the external surfaces of the fertilizer. Other efforts have been directed at increasing the acidity of the soil surrounding a plant or seed in order to promote Zn uptake. For example, U.S. Pat. No. 8,221,515 refers to the application of a powdered micronutrient in addition to a powdered acidifying agent to an agronomic carrier (i.e., a seed or fertilizer granule) to increase soil acidity and promote Zn uptake.
Incorporating micronutrients like Zn into fertilizers (e.g., through chelation or by forming complexes with macronutrients) can offer protection from adsorption or precipitation, especially in neutral and alkaline soils, thus increasing the availability of the micronutrient. Therefore, there remains a need for methods and compositions that can increase the solubility and availability of Zn already present in fertilizer complexes and formulations.